To use nitride semiconductors in semiconductor devices, in particular optoelectronic devices, has received considerable attention for a relatively long time not at least due to the potential possibility to achieve components active in wavelength areas not accessible with conventional semiconductor materials. In the 1990's, two breakthroughs have been made on nitride semiconductor growth: the fabrication of high-quality GaN films and the realization of p-type GaN. Following these, blue and green LEDs and Laser diodes have been commercialized, and UV LED began to be reported based on AlN. Nitride based semiconductors are also of interest for transistors and other electronics in high voltage and high temperature applications.
The GaN films are typically grown by industrial scale MOCVD techniques. To achieve acceptable quality of the films the growth is performed with high precursor flow such as NH3 and TMG (trimethylgallium), and hence high partial pressures. A commonly used measure is the so called “V/III-ratio” which relates the molar flow of the precursor elements, for example the molar ratio between the NH3 and TMG. The V/III-ratio used for GaN film growth is in the range of 1000-10000.
Top standard GaN films of today do still have very high densities of defects, however. Under such background, 1-dimensional structures, that is nanowires based on nitrides have attracted plenty of attentions from researchers. Several methods such as VLS, template-confinement growth, and oxide-assisted growth have been reported for GaN nanowires growth.
Selective area growth of GaN has also been studied extensively from 1990's to reduce the dislocations density in GaN films. From dot-patterned GaN openings, Akasaka et al. showed GaN columns growth with the diameter of 5 μm [1]. Recently, Hersee et al. reported array fabrication of GaN wires sized by 221 nm using selective area growth. It is described that pulsed growth had to be used for growing GaN nanowires to confine the lateral growth [2]. Pulsed growth is also referred to as migration enhanced growth. The method may be described as a two step method comprising an initial nanowire growth step referred to as a selective growth step wherein both precursor gases are provided. The initial growth step is followed by a secondary step of pulsed growth, wherein precursor gases are provided one at the time.